An Enhanced Zebrafish Xenograft Model to Study the Behavior of Human Hematopoietic Stem Cells and Patient Derived Leukemia In Vivo

Human hematopoietic stem cells (HSCs) are characterized by their unique ability to self-renew and differentiate into multiple cell types. Transplantation of human HSCs into immunocompromised mice is the gold standard for evaluating the in vivo repopulation capacity of these cells and has been adapted to evaluate the leukemic potential of leukemia-initiating cells (LICs) in acute leukemia. In spite of the popularity of this approach, certain cellular subsets do not survive post xenotransplantation as they are dependent on human factors that may be missing in the murine microenvironment. We pioneered human leukemia xenografts in the zebrafish model (Corkery et al., 2011; Bentley et al., 2015) as a parallel strategy, taking advantage of the immuno-permissiveness and transparency of zebrafish larvae to provide unprecedented continuous observation of leukemia evolution. However, as in mice, the addition of human cytokines and growth factors may further improve engraftment and optimize the utility of the zebrafish as a preclinical transplant model for medium to high-throughput therapeutic screening by creating a microenvironment more comparable to that found in patients.

Using transgenic technology, we created novel zebrafish lines that express human stem cell factor (SCF/KITLG), granulocyte-macrophage colony stimulating factor (GM-CSF/CSF2), and stromal cell-derived factor 1 alpha (SDF1α/CXCL12). All of these factors are essential for the survival and expansion of HSCs, progenitors and LICs. CMK, GRWi (Down Syndrome- acute myeloid leukemia (DS-AML)) and Jurkat (T-acute lymphoblastic leukemia) cell lines were injected into the yolk sac of these multi-cytokine zebrafish embryos. These cells engrafted and exhibited increased cell proliferation and physiological migration compared to control casper embryo transplants. Moreover, only in the multi-cytokine fish did these cells home to hematopoietic niches, including the caudal hematopoietic tissue (CHT) (equivalent to the fetal liver) at 7 days-post injection (dpi) and the kidney marrow at 11 dpi.

The yolk sac is not the physiological site for hematopoietic cell growth and thus we injected patient-derived AML cells into the circulation of the multi-cytokine fish. We observed similar trends in proliferation and migration as seen with leukemia cell lines. Historically, human HSCs do not survive beyond 24h in zebrafish xenografts (Pruvot et al. 2011). Given our success with patient-derived leukemia samples, we wanted to see if our multi-cytokine fish would provide a more favorable environment for human HSC survival. Indeed, primary human umbilical cord blood-derived HSCs were injected into the circulation of zebrafish larvae and survived past 3 dpi in the multi-cytokine fish but not in casper control larvae.

Transgenic zebrafish expressing human cytokines provide an enhanced model for studying human leukemia and stem cell biology in a visually tractable system that more closely recapitulates the human microenvironment. The inherent capacity of the zebrafish for rapid and efficient chemical screening will be exploited in this model for preclinical testing of anti-leukemic therapies and to evaluate HSC enhancing agents in vivo .